John,
You are correct that drag is the number one cause of a refusal.
I'm pretty sure you have also been in the situation where you cast a fly that brings the fish right up to the fly before it refuses. You cast again and the fish comes up half way and refuses. The on the third cast, there is no reaction to the fly.
In that case, I am convinced that it not drag that caused the refusal. It is the fly. If the fly was correct, the fish would come up to the about same point as before because that was when it would notice drag. Instead it comes up until it can recognize that it is "not food" and refuses much, much earlier, before it could notice the micro drag. On the third try it does not even have come and investigate the fly. It has learned that the "impression" of the fly is not food.
Regards,
Silver
"Discovery consists of seeing what everybody has seen and thinking what nobody has thought"..........Szent-Gyorgy